The last time I tried reading Japanese using Koreader, it took the vertical right to left text and rendered it left to right and horizontal, but sometimes with only a couple of characters to a line. It also couldn't handle the furigana.

So now, I only use Nickel to read Japanese texts.

Chinese texts which are written in a left to right format are handled very well by Koreader. And so I do my Chinese reading with Koreader because it is possible to install a wider range of Chines language dictionaries in Koreader than it is in Nickel.

Hopefully, Koreader will also eventually be able to handle vertical text and furigana. It would be nice to have the freedom to install extra dictionaries like I can for Chinese.